1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly for optimization of dielectric circuit board materials for improved performance in quarter-wave transformers.
2. Description of the Related Art
RF circuits and quarter-wave transformers are commonly manufactured on specially designed substrate boards. For the purposes of RF circuits, it is important to maintain careful control over impedance characteristics. If the impedance of different parts of the circuit do not match, this can result in inefficient power transfer, unnecessary heating of components, and other problems. A specific type of transmission line often used to match the impedances of different parts of the circuit is a quarter-wave transformer. Hence, the performance of quarter-wave transformers in printed circuits can be a critical design factor.
As the name implies, a quarter-wave transformer typically has an electrical length precisely λ/4, where λ is the signal wavelength in the circuit. The proper characteristic impedance of a quarter-wave transformer is given by the formula Z0=√{square root over (Z1Z2)}, where Z0 is the desired characteristic impedance of the quarter-wave transformer, Z1 is the impedance of a first transmission line to be matched, and Z2 is the impedance of a second transmission line or load being matched to the first transmission line.
Printed quarter-wave transformers used in RF circuits can be formed in many different ways. One configuration known as microstrip, places the quarter-wave transformer on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the quarter-wave transformer is covered with a dielectric substrate material. In a third configuration known as stripline, the quarter-wave transformer is sandwiched within substrate between two electrically conductive (ground) planes.
Two critical factors affecting the performance of a substrate material are permittivity (sometimes called the relative permittivity or εr) and the loss tangent (sometimes referred to as the dissipation factor). The relative permittivity determines the speed of the signal, and therefore the electrical length of transmission lines and other components implemented on the substrate. The loss tangent characterizes the amount of loss that occurs for signals traversing the substrate material. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to √{square root over (L1/C1)} where L1 is the inductance per unit length and C1 is the capacitance per unit length. The values of L1 and C1 are generally determined by the physical geometry and spacing of the line structure as well as the permittivity and permeability of the dielectric material(s) used to separate the transmission line structures. Conventional substrate materials typically have a relative permeability of approximately 1.0.
In conventional RF design, a substrate material is selected that has a relative permittivity value suitable for the design. Once the substrate material is selected, the line characteristic impedance value is exclusively adjusted by controlling the line geometry and physical structure.
The permittivity of the chosen substrate material for a transmission line, passive RF device, or radiating element influences the physical wavelength of RF energy at a given frequency for that line structure. One problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is optimized for all of the various passive components, radiating elements and transmission line circuits to be formed on the board. In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide respectively for practical implementation for a given substrate. Since the physical size of the microstrip or stripline is inversely related to the relative permittivity of the dielectric material, the dimensions of a transmission line can be affected greatly by the choice of substrate board material.
An inherent problem with the foregoing approach is that, at least with respect to the substrate material, the only control variable for line impedance is the relative permittivity, εr. This limitation highlights an important problem with conventional substrate materials, i.e. they fail to take advantage of the other factor that determines characteristic impedance, namely L1, the inductance per unit length of the transmission line.
Conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the permittivity. Accordingly, conventional dielectric substrate arrangements for RF circuits have proven to be a limitation in designing circuits that are optimal in regards to both electrical and physical size characteristics.